Modern medical ultrasonic diagnostic systems usually can combine detecting contents and display them simultaneously, for example, synchronously displaying two-dimensional B mode image and Doppler spectrum diagram, or synchronously displaying Doppler spectrum diagram and colour blood flow image, by which a doctor diagnoses diseases.
FIG. 1 shows a typical medical ultrasonic imaging system for measuring Doppler blood flow velocity. The system comprises a multi-element ultrasonic transducer array (not shown), which is capable of converting high voltage electrical pulses into ultrasonic waves in transmitting stages and converting ultrasonic echoes into electric signals in receiving stages. The echo signals received by each of the array units of the transducer are sent to a beamformer module, and are processed to improve the signal-to-noise ratio of the echo signals. Then, according to the characteristics of the echoes, the system sends output signals of said beamformer to respective signal processing modules, wherein those related to the B mode images are sent to a two-dimensional B mode signal processing module for obtaining two-dimensional B mode image data; those related to spectral Doppler information are sent to the spectral Doppler signal processing module for obtaining Doppler spectral data; and those related to colour blood flow are sent to the colour blood flow signal processing module for obtaining colour blood flow data. Finally, a display module combines said two-dimensional B mode image data, Doppler spectral data and colour blood flow data to form resultant data for synchronously displaying on the display screen.
FIG. 3 shows a flowchart of processing Doppler spectral signals, for example, in an ultrasonic Doppler blood flow analysis system. After the beamforming of the ultrasonic echo, an RF echo is formed, which is decomposed into two component signals by a demodulation module, i.e., an in-phase component I signal and a quadrate component Q signal. Then, in a continuous wave Doppler system, the I and Q components are directly in a wall filtering processing stage. In a pulsed wave Doppler system, the I and Q components are gated in range first, respectively, that is, accumulated in a specific time interval. The accumulation time interval and the length of the pulsed Doppler transmitted pulse are selected by an operator according to actual situations, then the I and Q components are in the wall filtering processing stage. The wall filter is a high pass filter, and can filter clutters caused by stable or slow moving tissues. The I and Q components after the processing of this stage, which mainly comprise the echoes caused by the motion of red blood cells, are sent to a power spectrum estimation module, which estimates a power spectrum usually by the use of fast Fourier transform (FFT). The number of points of the FFT may be 128 or 256. Since the dynamic range of the estimated power spectrum is too wide, it is necessary for the estimated power spectrum to be compressed into a gray scale display range. The Doppler spectrum diagram finally displayed on the screen represents the power spectral intensity at a certain time and at a certain velocity, i.e. at a certain frequency shift. The system may further comprise an automatic envelope detection module for analyzing the compressed data, automatically tracking the variations of the peak velocity and mean velocity of the blood flow with time, and displaying them on the Doppler spectrum diagram in real time. Furthermore, the wall-filtered I and Q data may further be sent to an acoustic processing module to form acoustic data of the forward blood flow and the reverse blood flow, then these data are D/A converted and sent to a speaker, respectively, so as to generate sounds of forward and reverse blood flows.
FIG. 2 shows a two-dimensional B type image and a spectral Doppler diagram synchronously displayed by the system. The upper portion of the figure indicates the two-dimensional B type image in which the dotted lines show the positions and directions of blood vessels. The operator may select sampling lines corresponding to the blood flow to be detected and corresponding interested regions. The lower portion of the figure indicates the Doppler spectrum diagram of blood flow in the selected region.
In order to display synchronously the two-dimensional B mode image and spectral Doppler image on the screen, as shown in FIG. 2, the ultrasonic imaging system usually performs fast switching between the two-dimensional B mode scanning and Doppler scanning. Thus, the B mode image scanning and the Doppler blood flow measurement scanning are performed in different time intervals. The fast switching between two different scanning modes has the advantage that the interaction of the imaging results is very small. Because the scanning of the two systems are performed in different time intervals, the two systems can share a single scanhead, and the Doppler transmitting mode may be a pulsed or a continuous mode. However, there is also an inherent drawback in this approach, that is, the Doppler signal may be missed due to interruption of the Doppler scanning while performing B type image scanning. The update of two-dimensional B type image and Doppler diagram implemented in this manner must result in discontinuity in the Doppler spectral diagram and Doppler sound. The discontinuous time interval of the Doppler signal caused by the switching to other working modes is called a gap.
Referring to FIG. 2, the solid lines correspond to the spectrum diagram in non-gap time intervals, and the dotted lines correspond to the spectrum diagram in gap time intervals. There is no Doppler signal in the gap time intervals; and the Doppler spectrum diagram is interrupted, that is, there is no Doppler spectrum in the time intervals of the dotted lines. In the gap time intervals, the Doppler sound is also interrupted due to lack of Doppler signals. Therefore, in a multimode ultrasonic scanning synchronous display system, a gap filling method is usually used to compensate visual or audio discontinuity of the Doppler spectrum diagram or Doppler sound caused by the gaps. As illustrated by the dotted line blocks and the flowchart in FIG. 3, the function of the gap filling module is to estimate the lost Doppler signals and to make the Doppler signals continuous, thereby the continuity of the Doppler spectrum diagram and the Doppler sound can be maintained.
In the technical solution disclosed in U.S. Pat. No. 5,476,097, Robinson proposed a method of filling the gaps of the results of the Doppler spectrum diagram and the Doppler sound after Doppler processing, so as to make them more continuous on visual or audio effect. The gaps in the Doppler spectrum diagram are filled by the use of a spanning method; each of the calculated spectra near the gaps is repeated for two times; and the gaps of the Doppler sound are filled by the use of directly repeating the Doppler sound results in the non-gap intervals.
In the technical solution disclosed in U.S. Pat. No. 4,559,952, Angelsen proposed a method of directly filling I and Q Doppler signals. According to that method, the I and Q Doppler signals before the gap are repeated in the gap period, the I and Q Doppler signals are continuous in the succeeding Doppler processing.
The above-mentioned prior art references have some disadvantages. The effect of sound filling of the filling method of U.S. Pat. No. 5,476,097 is relatively ideal, no interrupt can be felt audibly. However, the visual effect of the Doppler Spectrum diagram filling is not satisfactory. The effect will be better where the difference of Doppler spectrum diagrams before and after the gap is not great. However, when the difference of the spectrum diagrams before and after the gap is relatively great, a significant sudden change can be seen at the joints of spanning due to the use of the spanning method. The filling method of U.S. Pat. No. 4,559,952 only employs the I and Q Doppler signals for filling. The gap interval may not be too long in order to result in a good filling effect. However, a too short gap interval may restrict the imaging quality of scanning of other modes.